Only add line items if calculations match order total calculations

Include tax in shipping cost calculations and remove tax from order total calculations
Add order total module calculations to Instant Update
Increase Instant Update callback timeout to 6 seconds
Only add TAXAMT if DISPLAY_PRICE_WITH_TAX is false
Add message stack output to checkout confirmation page
If PayPal transaction total does not match order total, redirect the customer to the checkout confirmation page when returning back to store (otherwise process the order). Add a notice to the checkout confirmation page to confirm the order.